How to redirect 'print' output to a file?

I want to redirect the print to a .txt file using Python. I have a for loop, which will print the output for each of my .bam file while I want to redirect all output to one file. So I tried to put:

f = open('output.txt','w')
sys.stdout = f

at the beginning of my script. However I get nothing in the .txt file. My script is:

#!/usr/bin/python

import os,sys
import subprocess
import glob
from os import path

f = open('output.txt','w')
sys.stdout = f

path= '/home/xxx/nearline/bamfiles'
bamfiles = glob.glob(path + '/*.bam')

for bamfile in bamfiles:
    filename = bamfile.split('/')[-1]
    print 'Filename:', filename
    samtoolsin = subprocess.Popen(["/share/bin/samtools/samtools","view",bamfile],
                                  stdout=subprocess.PIPE,bufsize=1)
    linelist= samtoolsin.stdout.readlines()
    print 'Readlines finished!'

So what's the problem? Any other way besides this sys.stdout?

I need my result look like:

Filename: ERR001268.bam
Readlines finished!
Mean: 233
SD: 10
Interval is: (213, 252)

Solution

The most obvious way to do this would be to print to a file object:

with open('out.txt', 'w') as f:
    print('Filename:', filename, file=f)  # Python 3.x
    # print >> f, 'Filename:', filename   # Python 2.x

However, redirecting stdout also works for me. It is probably fine for a one-off script such as this:

import sys

orig_stdout = sys.stdout
f = open('out.txt', 'w')
sys.stdout = f

for i in range(2):
    print('i = ', i)

sys.stdout = orig_stdout
f.close()

Since Python 3.4 there's a simple context manager available to do this in the standard library:

from contextlib import redirect_stdout

with open('out.txt', 'w') as f:
    with redirect_stdout(f):
        print('data')

Redirecting externally from the shell itself is another option, and often preferable:

./script.py > out.txt